1. Field
Embodiments described herein relate generally to a semiconductor memory.
2. Background Art
In recent years, semiconductor memories including resistance change elements serving as memory cells have gained attention as next-generation memories. Such semiconductor memories include a magnetoresistive random access memory (MRAM) having magnetoresistive elements serving as memory cells, a phase change random access memory (PRAM), and a resistive random access memory (ReRAM).
For example, in an MRAM, a memory element is a magnetoresistive element called a magnetic tunnel junction (MTJ) element.
The MTJ element includes a fixed layer (pin layer) in which a direction of magnetization is fixed by an antiferromagnetic layer, a free layer (recording layer) in which a direction of magnetization can be freely inverted, and an insulating layer (barrier layer) interposed between the fixed layer and the free layer. In the MTJ element, a resistance change called a magnetoresistance effect is utilized. The resistance change is caused by a direction of magnetization of the free layer relative to the fixed layer.
In other words, data is identified as “1” or “0” using a resistance difference caused by a relative direction of magnetization.
Particularly, spin injection MRAMs have recently received attention, in which flux reversal made by polarized spin current injection is used for a writing scheme. In a spin injection MRAM, a current amount required for flux reversal (inversion threshold current) is specified by the density of current passing through an MTJ element.
Furthermore, a writing operation is performed on the spin injection MRAM by applying a writing current not lower than the inversion threshold current.
Moreover, the polarity (“0” or “1”) of data is determined by the injection direction of the writing current to the MTJ element.
For example, in the case where a certain piece of information is written in the MTJ element, an excessive current flows in one writing direction relative to an inversion current and a writing current in the other direction fails to meet the inversion current due to an easy direction of flux reversal of the MTJ element and the positional relationship between a cell transistor and the MTJ element.
In other words, characteristics vary depending on the writing direction.
Thus, unfortunately, an excessive load may be applied to the MTJ element by an excessive current in one writing direction and an insufficient writing current in the other direction may reduce a writing margin.